An authenticity mark of the above mentioned type known from German Patent Specification DE 2,829,778.
In order to make it extremely difficult for unauthorized persons to copy a document such as a credit card, driving license, passport, etc, it is generally proposed that an original document should be provided with an "authenticity mark" which is difficult to copy. The authenticity mark described in the above-mentioned German Patent Specification DE 2,829,778 is formed by fluorescent granules applied to the document. These granules are disposed randomly within a linearly extending surface area of the document. If this line of fluorescent granules is illuminated by a suitable source of radiation and the line is simultaneously scanned by a suitable scanning device, the received fluorescent radiation will cause the scanning device to provide an electrical signal which will vary in amplitude in accordance with the random pattern of granules within the linearly delimited area. The varying signal is considered unique and can be used to check the authenticity of the document.
A first disadvantage of this prior art authenticity mark is the fact that the intensity of the light, received by the scanning device, will show only rather minor variations because every section of the fluorescent line, scanned by the scanning device, comprises fluorescent material. Only in case rather coarse granules are used with rather large mutual separations between each of these granules, a more pronounced intensity variation will result. The use of rather coarse granules, however, certainly if these granules are separated by rather extended intermediate void areas, gives rise to an easy falsification possibility of the document, because it is rather easy the manipulate coarse single granules with normally available means.
A second disadvantage of this prior art authenticity mark results from the fact that the intensity level of the light received from an area covered by a fluorescent granule and the intensity level of the light, received from an area, not covered by a fluorescent granule, both are comparatively low and of the same magnitude as the environmental light level. In other words, the functioning of the scanning device can be disturbed rather easily by noise or radiation from the environment. This noise problem might be reduced by using specific fluorescent granules reacting only on a very specific radiation frequency, preferably outside the frequency band of the environmental radiation, in combination with a scanning device which is sensitive only to the very specific radiation frequency. Such a combination, however, is rather costly and certainly not suited for broad scale application of the authenticity mark on documents such as for instance credit cards, passports, driving licenses, identity cards, admission passes, etc.
According to further prior art it is also possible to make an authenticity mark by embedding magnetic particles or magnetic fibers within a substrate in a random pattern. In this respect the attention is for instance drawn to DE 2.635.795 an inherent problem with the detection of such particles or fibers is that a magnetic detection head has to be used for detecting these fibers and this head has to be guided along an accurately predetermined track. Any mis-alignment between the magnetic head and the predetermined track will result into a non validity decision of the authenticity mark. Various proposals are described in the prior art literature for solving this problem.